Reliability-related issues of integrated circuits have existed for decades. In the past, negative-bias thermal instability (NBTI) related issues have not had a significant impact on designs due to the relatively large size of integrated circuits. With the scaling of integrated circuits, however, the NBTI related issues become increasingly important. For designs using 0.13 micron technologies or below, NBTI reliability analyses need to be taken into consideration of circuit design in order to maximize circuit performance, particularly for circuits demanding high reliability and/or performance. Identifying reliability-related performance problems in early design stages means faster and more reliable end products with significantly reduced chip re-spins, wasted silicon, and potentially millions of dollars.
One example of NBTI related issues is the degradation of Vccmin of static random access memory (SRAM) cells under NBTI stresses, such as high voltage and/or high temperature. Vccmin is the higher of the minimum voltages required to read data from, and write data to, SRAM memory cells. Because Vccmin affects both the performance and reliability of SRAM cells, and because the Vccmin drifts under NBTI influence, the prediction of the Vccmin's behavior under NBTI stress becomes very important for the design of SRAM memory.
Existing modeling and simulation methods do not provide means for designers to predict the behavior of integrated circuits under NBTI influence. For example, in SPICE simulation for determining the Vccmin of an SRAM cell, circuit parameters of the SRAM cell are first inputted into the SPICE, and then static noise margins (SNM) and write margins (WM) for the nodes of the SRAM cell are determined by SPICE. Read Vccmin and write Vccmin are then calculated accordingly. The Vccmin of the SRAM cell is the greater of the read Vccmin and write Vccmin.
The conventional methods only provide one Vccmin value for each simulated SRAM cell. The behavior of SRAM array, which includes multiple SRAM cells, can't be provided. In addition, the SPICE simulation does not provide the behavior of the SRAM(s) under NBTI stress, and thus designers lack the means for predicting the behavior of the memory array in order to optimize design.
Therefore, a solution is needed for designers to identify possible problems such that the designer can design to accommodate around these situations, such as Vccmin drift or voltage overshoot, for which the designers may compensate for without sacrificing performance of integrated circuits.